This invention relates to a developing unit for an electrostatic recording apparatus such as an electrophotographic copying apparatus.
An electrostatic recording apparatus such as an electrophotographic copying apparatus usually uses a rotary drum type photosensitive member having, as its outer circumferential face, a photoconductive layer. Such an electrostatic recording apparatus obtains desired records by charging the photosensitive member, exposing an image to form an electrostatic latent image of an original image on the above mentioned photorecepting layer while rotating the photosensitive member, developing the image with toner, and transferring the image onto a recording paper.
In the developing unit used for electrostatic recording apparatus such as an electrophotographic copying apparatus, a developing sleeve 3 which is provided with a built-in fixing magnet faces photosensitive drum 1 in a housing 2 which faces such a photosensitive drum, as shown in FIG. 12. While transferring the developer which is mixed in the housing 2 onto the peripheral surface of the developing sleeve 3, toner particles which are contained in the developer are electrostatically absorbed on the electrostatic image which is formed on the surface of the photosensitive drum 1.
In other words, the electrostatic image is made into a visible image by the toner. The distance d between this developing sleeve 3 and the photosensitive drum has been kept constant. If the distance d is not uniform, generated electric field becomes uneven, which adversely affects developability. Therefore, a roller 4 which is processed to the size equivalent to the distance d has been provided on the developing sleeve 3 at the end of the axis of the developing sleeve 3.
The housing 2 has been pressed toward the photosensitive drum 1 in the direction shown with an arrow mark by a means such as spring (not shown in the figure) so that the roller 4 is in contact with the surface of the photosensitive drum 1.
In the case of the above developing unit, however, precision of processing of the roller is required. Since the roller 4 rotates while being always in contact with the photosensitive drum 1, the surface of the drum tends to be damaged.
Recently a unit of entire image formation section including a photosensitive drum and a developing unit has been developed. The unit is replaced for each color to make color copies easily. On such an unit, a photosensitive drum and an entire developing unit are installed in one housing. However, since a distance d between the surface of the photosensitive drum and the developing sleeve must be secured, it was impossible to press the developing sleeve alone against the photosensitive drum independent from a developer mixing plate, etc.